Please refer to FIG. 1, which is the schematic circuit diagram of a conventional boost circuit 1 in the prior art. In FIG. 1, the boost circuit includes a DC/DC boost converter 11 and a feedback control circuit 12. The feedback control circuit 12 further includes a voltage/current detector 121 for receiving an input voltage feedback signal SVi, an output voltage feedback signal SVo and an input current feedback signal SIi generated by the DC/DC boost converter 11, an oscillator 122, a modulator 123, a soft-starting unit 124 and a feedback controller 125. In which, an input voltage Vin turns into an output voltage Vo after a boost, and the output voltage Vo is controlled by an output signal K of the feedback controller 125. As shown in FIG. 2, the variations of the output voltage Vo of the boost circuit 1 is increasing from an initial value S to a peak value P gradually and then decreasing to a stable value Vo slowly. The operational principles of the boost circuit 1 are described as follows. The voltage/current detector 121 coverts the detected signal into a voltage signal and outputs the voltage signal to the modulator 123 firstly. The modulator 123 modulates the time period of a pulse train according to the voltage signal and a first output signal generated by the oscillator 122 secondly. The modulated pulse train is outputted from the modulator 123 to the feedback controller 125 and compared with the first output signal generated by the oscillator 122 and a second output signal generated by the soft starting unit 124 thirdly. And the output signal K is generated and outputted by the feedback controller 125 to control the switching of a switch Q1 of the boost circuit 1 so as to generate the boost lastly.
According to the waveform of the output voltage Vo as shown in FIG. 2, the value of the output voltage Vo is rising from an initial value S to a peak value P and then decreasing from P to a stable value Vo. Since the conventional boost circuit 1 will make the output voltage Vo rise to the highest value of P, relatively the electronic elements of the boost circuit 1 such as the switch Q1, the diode D and the output capacitor C1 must have the higher withstand voltage and the higher reliability, and the manufacturing costs of these electronic elements are higher. The power consumption of the conventional boost circuit 1 is relatively higher and more expensive since the output voltage Vo will raise to the peak value P, which is higher than the rated voltage Vo.
Keeping the drawbacks of the prior arts in mind, and employing experiments and research full-heartily and persistently, the applicant finally conceived the configuration and the controlling method of a boost circuit having a pulse-width modulation limiting controller.